Gordiid worms

Gordiid worms (Phylum Nematomorpha: Class Gordiida) usually parasitize crickets and other arthropods. They are not considered parasites of people. Yet they occasionally emerge from the human body—in vomit or feces, for example. In such cases, scientists believe, the human has accidentally ingested the worm, but it has not succeeded in colonizing the human body. For example, a person might swallow some gordiid-ridden insect that has found its way into his food.

What earns the gordiid a mention on this site is its sometimes painful way of exiting the human body. It can cause abdominal pain in its attempts to escape, for example. It has been known to crawl out of the tip of the penis, and circumstantial evidence suggest it may emerge from the female urethra as well. How it finds its way into the urinary tract of humans is a matter of some dispute.